The battery has become a primary power source for many portable electronic devices such as radios, hearing aids, watches, calculators and the like. In order to maintain the overall electronic device as compact as possible the electronic devices are usually designed with cavities to accommodate miniature cells as their source of power. The cavities are usually made so that a cell can be snugly positioned therein thus making electronic contact with appropriate terminals within the device. A major potential problem in the use of a high energy density cell such as a divalent silver oxide/zinc/alkaline cell is that if the cell bulges, it usually becomes wedged within the cavity of the device which sometimes can result in damage to the device. In addition, when the cell bulges it may disturb the seal whereupon the electrolyte might escape to cause damage to the device and/or oxygen from the atmosphere may enter which could cause wasteful corrosion of the anode. On the other hand, if the seal of the cell is maintained, high internal gas pressure may develop which could cause not only bulging of the cell but even possible disassembly of the cell.
Although divalent silver oxide is a good high capacity positive active material when used in alkaline cells, it is rather unstable when in contact with an aqueous alkaline electrolyte. Specifically, divalent silver oxide is a highly oxidizing material and as such it is capable of reacting in an aqueous alkaline electrolyte to yield oxygen gas. In addition, divalent silver oxide will liberate oxygen when it decomposes to form monovalent silver oxide when in contact with the aqueous alkaline electrolyte. Divalent silver oxide can also attack cellulosic materials in the cell, such as the separator, to form carbonate ion at the expense of even more electrolyte. These are undesirable processes because they lead to bulging of the cell, deterioration of its parts and loss of service.
U.S. Pat. No. 3,853,623 discloses one approach to stabilize divalent silver oxide in a silver oxide/zinc/alkaline cell through the use of gold ions incorporated into the alkaline electrolyte on the positive side of the cell's separator or gold oxide added to the positive active material of the cell.
U.S. Ser. Nos. 891,823 and 891,824 both filed on Mar. 30, 1978 disclose an alkaline silver oxide cell employing a divalent silver oxide-containing electrode wherein either a cadmium compound or zinc oxide additive, respectively, is incorporated in the cell to improve the chemical stability of the divalent silver oxide when in contact with the cell's alkaline electrolyte.
U.S. Pat. No. 4,015,056 discloses a method for manufacturing a stable divalent silver oxide depolarizer mix wherein the mix is treated with a mild reducing solution of a reducing agent such as methanol followed by a treatment with a strong reducing solution of a reducing agent such as hydrazine to form a layer of silver on the surface of the depolarizer mix.
U.S. Pat. No. 4,009,056 discloses a primary alkaline cell having a stable divalent silver oxide depolarizer mix comprising a negative electrode, a divalent silver oxide depolarizer mix, a separator between said negative electrode and depolarizer mix, and an alkaline electrolyte and wherein the surface of the depolarizer mix is treated with a mild reducing solution to form a reduced layer surrounding the mix and the surface of the reduced layer adjacent to the separator is coated with a layer of silver. It is further disclosed that the reduced layer surrounding the depolarizer mix in combination with the layer of silver provides improved stability of the depolarizer mix in the alkaline electrolyte and a single voltage plateau during discharge of the cell.
U.S. Pat. No. 4,048,405 discloses a high drain rate, primary alkaline cell comprising a negative electrode, a divalent silver oxide/monovalent silver oxide depolarizer blend, a separator between said negative electrode and depolarizer blend, and an alkaline electrolyte consisting essentially of potassium hydroxide and wherein the surface of the depolarizer blend adjacent to the separator is coated with a layer of silver. It is stated that the reduced surface layer of the depolarizer provides improved stability of the depolarizer blend in the alkaline electrolyte, and it provides the cell with a single voltage plateau during discharge.
It is an object of the present invention to provide a divalent silver oxide-containing cell in which the internal gas pressure buildup is reduced so as to effectively eliminate distortion of the cell's housing.
It is another object of the present invention to provide a divalent silver oxide cell wherein the surface of the divalent silver oxide-containing electrode that faces the separator is substantially reduced to monovalent silver oxide so as to effectively control gas pressure buildup within the cell and thereby effectively eliminate distortion of the cell's housing.
It is another object of the present invention to provide a divalent silver oxide cell wherein a major portion (over 50 percent) of the surface of the divalent silver oxide-containing electrode that faces the separator is in-situ reduced to monovalent silver oxide so as to effectively control gas buildup within the cell and thereby effectively eliminate distortion of the cell's housing.
It is another object of the present invention to provide a method for producing a divalent silver oxide-containing cell in which the surface of the divalent silver oxide-containing electrode facing the separator is in-situ reduced by a mild reducing agent to form monovalent silver oxide so as to effectively control gas buildup within the cell and thereby effectively eliminate distortion of the cell's housing.
The foregoing and additional objects will become more fully apparent from the following description.